Color display device

ABSTRACT

In a plasma display or a field emission display, sub-pixels having extra (non-saturating) phosphors are applied to enhance the efficacy. Depending on the luminance and color of a pixel to be displayed, driving of the most efficient combination of sub-pixels is performed.

BACKGROUND OF THE INVENTION

The invention relates to a color display device comprising a firstsubstrate provided with a layer of fluorescent material, and a second,transparent substrate, said color display device comprising means foraddressing pixels during use. The display device may be a plasma displaydevice but also, for example, a display device of the field emissiontype. The address electrodes may be present on both the first and thesecond substrate in the case of a plasma display device. Dependent onthe type of display device, the fluorescent material is patterned or notpatterned.

A display device of this type is used, inter alia, in large-area, flatdisplay screens, for example, for HDTV.

A color display device (plasma display panel or PDP) of the typementioned above is described in EP-A-0 488 891. This applicationdescribes measures for obtaining grey values or color gradations. Tothis end, the frame time is divided into a plurality of sub-frameshaving a weighted duration (for example in the ratio 1:2:4: . . . :128).When a color, for example green, is fully driven, a frequently usedphosphor such as Zn₂SiO₄:Mn (willemite) exhibits saturation phenomena.The efficiency of the phosphor (number of emitted photons/number ofincoming photons) deteriorates, so that the brightness decreases.

OBJECTS AND SUMMARY OF THE INVENTION

It is, inter alia, an object of the present invention to provide adisplay device of the type described, which has an optimum brightness orluminance, also when a given color is fully driven.

To this end, a color display device according to the invention ischaracterized in that the layer of fluorescent material comprises atleast two different phosphors of substantially the same color at thelocation of a pixel.

Substantially the same color is herein understood to mean that thedifference in spectral color of the emission peaks of the phosphors,measured as the distance in C.I.E co-ordinates (x, y co-ordinates) is atmost 0.35 (and preferably at most 0.25).

The invention is based on the recognition that the decrease of theefficiency for different phosphors depends in different manners on thenumber of incoming photons. By using two phosphors with a difference ina variation of the efficiency, it will be possible to mix colors ofphosphors which, as regards color are (slightly) different but exhibit adifferent behavior as regards loss of efficiency in the case of fulldrive (maximum brightness), or to choose between the two phosphors.

An optimum effect is achieved when one of the two phosphors is anon-saturating phosphor. A non-saturating phosphor is herein understoodto mean a phosphor in which the number of emitted photons per unit ofsurface and time has decreased at most 15% in the case of a driveyielding a brightness (or luminance) of 500 Cd/m² as compared with adrive yielding a brightness of 10 Cd/m². Upon excitation by means of anUV plasma which is maintained with alternating voltages, this meansthat, for example, at higher frequencies (at least up to 10 kHz andpreferably up to 100 kHz or more) the efficiency (number of emittedphotons/number of incoming photons) decreases by at most 15%. Afrequently used phosphor such as Zn₂SiO₄:Mn (willemite) has alreadysaturation phenomena from approximately 1 kHz in this application. Theefficiency (number of emitted photons/number of incoming photons) hasalready dropped to approximately 90% and decreases rapidly at higherfrequencies (to approximately 50% at 100 kHz).

However, for non-saturating phosphors, the efficiency remainssubstantially constant through a large frequency range.

Suitable non-saturating phosphors are, for example:

1) (Ce,Gd)MgB₅O₁₀:Tb, or CBT

2) (Ce,La,Gd)PO₄:Tb or LAP

3) (Y,Gd)BO₃:Tb.

A preferred embodiment of a display device according to the invention ischaracterized in that the layer of fluorescent material at the locationof a pixel, viewed in a direction transverse to the first substrate,comprises at least two sub-pixels having different phosphors ofsubstantially the same color for different sub-pixels. An incoming(video) signal can now be split up, for example, into two or moresub-signals each supplying one of the sub-pixels with a separate drivesignal. By means of the sub-signals, the extent of mixing of the colorsis then adjusted, or a choice is made between the sub-pixels.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a diagrammatic cross-section of a part of a plasma displaydevice;

FIG. 2 shows diagrammatically a part of a plasma display device;

FIG. 3 shows diagrammatically a distribution of phosphors across apixel; while

FIG. 4 shows the associated C.I.E. color triangle; and

FIGS. 5 to 8 show variations of FIG. 3.

The Figures are diagrammatic and not drawn to scale. Correspondingcomponents generally have the same reference numerals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a plasma display device 10, in this case an AC displaypanel (AC PDP or AC Plasma Display Panel), having a first substrate 1provided with two display electrodes 2 which are coated with adielectric layer 3 (for example, glass), and a second, transparentsubstrate 5 provided with fluorescent material 8. In this embodiment,the second substrate 5 is provided with address electrodes 6. In therelevant embodiment, a sub-pixel as defined by the address electrode 6and the display electrodes 2 is bounded by partition walls 7 which bounda discharge space. The partition walls 7 are not absolutely necessary,while display electrodes and address electrodes may be alternativelypresent on one substrate.

A gas discharge mixture 9 consisting, in this embodiment, of ahelium-xenon mixture, is present in the discharge space between thesubstrates 1, 5. Other mixtures are alternatively possible, such asneon-xenon, argon-xenon, krypton-xenon, argon-neon-xenon,argon-helium-xenon, krypton-neon-xenon, krypton-helium-xenon or mixturesthereof, the quantity of xenon being in a range between 5 and less than100%. As is known, UV radiation is generated in plasma display devices(plasma display panels or PDPs) in the discharge space at the area of a(sub-)pixel, which UV radiation causes the fluorescent material 8(phosphors) to luminesce. To this end, the display electrodes 2 aredriven, for example, from X and Y drivers 20, 21 and the addresselectrodes are driven from an A driver 22 (FIG. 2). To this end, anincoming signal 11 is stored in a frame memory 12 and in a sub-framegenerator 13. The required pulses are generated in the processing unit14 for the reset pulses, the ignition pulses and the sustain pulseswhich energize the display electrodes 2 via the X and Y drivers 20, 21,while addressing takes place via the A driver 22 controlled by anaddress generator 16. Mutual synchronization takes place via a timingcontrol circuit 15.

After a sub-pixel has been excited, the ignition is maintained by thesustain pulses across the display electrodes within a pixel. Dependenton the grey tint to be displayed, these are offered more frequently orless frequently per pixel. The sustain frequency therefore determines,together with the grey tint, the number of times a UV photon impingesupon a phosphor.

FIG. 3 is a diagrammatic front-elevational view of a display device,particularly the location of a number of phosphors 8, 8′, 8″, 8′″ withthe colors red (R), green (G₁), blue (B) and a second color green (G₂).The broken line 25 diagrammatically represents a pixel.

The C.I.E. color triangle of FIG. 4 shows the luminances associated withthese phosphors as B(Y₁), R(Y₂), G₁(Y₃) and G₂(Y₄) (Y: luminance) inthis color triangle. Generally, it holds for display of a pixel x_(o),y_(o) with luminance Y_(o) by means of three phosphors$Y_{o} = {\sum\limits_{i = 1}^{i = 3}{y\quad i}}$

in which${\sum\limits_{i = 1}^{i = 3}{\left( {x_{i} - x_{0}} \right)\left( {Y_{i}/y_{i}} \right)}} = {{0\quad {\sum\limits_{i = 1}^{i = 3}{\left( {x_{i} - x_{0}} \right)\left( {Y_{i}/y_{i}} \right)}}} = 0}$

In the embodiment of FIG. 4, the point Y₀ can be realized by weightedexcitation of the phosphors B, R and G₁ or by weighted excitation of thephosphors B, R and G₂. At an equal efficiency of all phosphors, such aweighting is not necessary (and an extra phosphor G₂ is in factredundant). Since the efficiency of the green phosphor willemite (withcolor co-ordinates x=0.25, y=0.67) denoted by G₁ in FIG. 4, rapidlydeteriorates at higher frequencies, the presence of a second phosphor G₂provides the possibility of such a choice, notably when a non-saturatingphosphor is chosen for G₂, for example, said CBT (with colorco-ordinates x=0.36, y=0.54). The co-ordinates (x_(o), y_(o)) are nowfound by means of the equations${\sum\limits_{i = 1}^{i = 4}{\left( {x_{i} - x_{0}} \right)\left( {Y_{i}/y_{i}} \right)}} = {{0\quad {\sum\limits_{i = 1}^{i = 4}{\left( {y_{i} - y_{0}} \right)\left( {Y_{i}/y_{i}} \right)}}} = 0}$

in which now ${\sum\limits_{i = 1}^{i = 4}Y_{i}} = Y_{0}$

in which simultaneously the efficiency$\eta_{0} = \frac{Y_{0}}{\sum\limits_{i = 1}^{i = 4}\frac{Y_{i}}{\eta_{i}}}$

may be maximal. The choice between the combinations Y₁, Y₂, Y₃ and Y₁,Y₂, Y₄ is now made by determining the maximal efficiency η_(o) by meansof Y₄=0, or η_(o) (Y₁, Y_(2,) Y₃), and by Y₃=0, or η_(o) (Y₁, Y₂, Y₄).The group of phosphors yielding the highest efficiency is then driven.

Similar calculations can be performed for points in the triangles, Y₂,Y₃, Y₄ and Y₁, Y₃, Y₄.

The choice between Y₃ (G₁) and Y₄ (G₂) is made in the device of FIG. 2by processing information about the color to be displayed, stored in theframe memory 12, in the processing unit 14. To this end, this unitcomprises, for example, a microprocessor or a look-up table in which thebehavior (notably the efficiency of the phosphors as a function of thefrequency (grey scale to be displayed) is stored. Dependent on the aboveconsideration, that phosphor is driven with which the desired color isobtained at the highest efficiency. To this end, the X and Y drivers aresupplied with drive signals for the relevant sub-pixels.

FIGS. 5 to 7 show a number of variants in which the phosphors aredistributed across sub-pixels in different ways.

In FIG. 7, a pixel 25 comprises three sub-pixels, the green sub-pixelcomprising a phosphor layer G_(m) which consists of a mixture ofwillemite and CBT.

Although the above-described discrimination between two differentphosphors cannot be made with drive signals, it appears that the mixtureis less rapidly saturated so that a higher efficiency is achievedthrough a large part of the color triangle.

In FIG. 8, extra blue and red phosphors (B₂, R₂) are added, which arepreferably non-saturating so that a good color adjustment goes togetherwith a maximum efficiency.

A completely different possibility is the use of a so-called whitephosphor instead of G₂ in FIGS. 3, 5, 6.

The invention is of course not limited to the embodiments describedhereinbefore. For example, the sub-pixels may have, for example,different surface areas, which may be incorporated in the data in theprocessing unit 14. The invention is also applicable to field emissiondisplay devices in which the efficiency of given phosphors is alsodependent on the luminance to be adjusted.

In summary, the invention relates to a luminescence-based color displaydevice having at least one extra phosphor per pixel, in which, dependenton the color to be displayed and the luminance (brightness), thecombination of sub-pixels to be driven is determined.

The invention relates to each and every novel characteristic feature andeach and every combination of characteristic features.

We claim:
 1. A color display device comprising a first substrateprovided with a layer of fluorescent material, and a second, transparentsubstrate, said color display device comprising means for addressingpixels during use, characterized in that the layer of fluorescentmaterial comprises at least two different phosphors of substantially thesame color at the location of a pixel.
 2. A color display device asclaimed in claim 1, characterized in that one of the two phosphors is anon-saturating phosphor.
 3. A color display device as claimed in claim1, characterized in that the layer of fluorescent material at thelocation of a pixel, viewed in a direction transverse to the firstsubstrate, comprises at least two sub-pixels having different phosphorsof substantially the same color for different sub-pixels.
 4. A colordisplay device as claimed in claim 3, characterized in that the displaydevice comprises drive means for converting an incoming signal into atleast two drive signals for separate sub-pixels.
 5. A color displaydevice as claimed in claim 4, characterized in that, dependent on thecolor to be displayed and the luminance of the pixel, the drive meansdetermine the drive of the sub-pixels.
 6. A color display device asclaimed in claim 1, or characterized in that the layer of fluorescentmaterial comprises at least two different green phosphors.
 7. A colordisplay device as claimed in claim 1, characterized in that the layer offluorescent material comprises a white phosphor.
 8. A color displaydevice as claimed in claim 1, characterized in that the first substrateis provided with address electrodes, and the second, transparentsubstrate is provided with at least two display electrodes having a gasdischarge mixture between the two substrates.